1. Field of the Invention
The present invention relates to surface acoustic wave devices for use as, for example, resonators and bandwidth filters, and more specifically, the present invention relates to a surface acoustic wave device including a dielectric thin film disposed on an elastic substrate and an IDT electrode or a reflector electrode disposed between the elastic substrate and the dielectric thin film.
2. Description of the Related Art
Surface acoustic wave devices generally used as bandwidth filters and other such filters for mobile communication equipment include at least one interdigital electrode (hereinafter referred to as an IDT electrode) including at least a pair of comb-like electrodes in contact with a piezoelectric component.
A piezoelectric substrate made of various piezoelectric materials is used for the piezoelectric component. Also, surface acoustic wave devices have been introduced in which a dielectric thin film having piezoelectric characteristics is disposed on an elastic substrate, such as a glass substrate or a piezoelectric substrate, and in which a dielectric thin film not having piezoelectric characteristics is disposed on a piezoelectric substrate. For example, a surface acoustic wave device having a dielectric thin film on an elastic substrate is disclosed in “Research for Mass Production of Surface Acoustic Wave Filters for TV and VTR using Zinc Oxide Piezoelectric Films” (Kadota, The 26th EM Symposium, 1997, pp. 83-90).
In general, surface acoustic wave devices having a known dielectric thin film have a transverse structure in which a plurality of IDT electrodes are disposed at predetermined intervals. The transverse structure allows strips of the IDT electrodes to reflect TTE (triple-transit echo), thus causing unwanted ripples. Accordingly, it is preferable to reduce the reflection at the strips of electrodes as much as possible to prevent the ripples. Therefore, double-strip type IDT electrodes, which do not cause acoustic reflection, and single-strip type IDT electrode in which reflection is reduced have been used.
On the other hand, various surface acoustic wave devices using unidirectional strips have recently been introduced to reduce the insertion loss thereof. These surface acoustic wave devices need to increase the reflection at the strips in order to increase the directionality thereof.
Also, surface acoustic wave devices having a resonator structure are disclosed in “Analysis of SAW Resonators Using Long IDT's and Their Applications” (IECE Transactions, 1977/9, Vol. J60-A, No. 9, pp. 805-812) and others. Reflectors used in these surface acoustic wave devices include strips capable of causing a large amount of reflection, so that they can give a desired amount of reflection even with a small number of strips. Also, the above-described “Analysis of SAW Resonators Using Long IDT's and Their Applications” teaches that increasing the amount of reflection at the strips of the multi-pair IDT disclosed therein leads to an IDT having a rapid conversion characteristic.
As described above, various surface acoustic wave devices which need to improve the reflection at the strips have been introduced. In these surface acoustic wave devices, the IDT electrode or the reflector electrode is disposed on the piezoelectric substrate.
On the other hand, in surface acoustic wave devices using the above-described dielectric thin film, more specifically, surface acoustic wave devices having a dielectric thin film having a piezoelectric property on a glass substrate or a piezoelectric substrate, and surface acoustic wave devices having a dielectric thin film of non-piezoelectric property on a piezoelectric substrate, the reflection coefficient of the strips of the IDT electrode or the reflector electrode is small. Therefore, it is difficult to improve the speed of the conversion by the IDT electrode and to reduce the size and the insertion loss of the surface acoustic wave device.